1. Field of the Invention
The present invention relates to the field of communications.
2. Prior Art
Various data communication systems modulate data onto inphase and quadrature components of a baseband signal and then mix those baseband components with inphase and quadrature components of an RF carrier to broadcast the modulated data in a single sideband of the RF carrier. Upon receipt of the broadcast signal, the reverse process is carried out, first receiving the broadcast signal, then downconverting to recover the inphase and quadrature components of the modulated baseband signal, and then recovering the data from those inphase and quadrature components.
In the prior art systems, techniques are known for recovering the data from the modulated signals, such as by use of one or more local oscillators. These techniques, in general, are sufficiently accurate to be relatively insensitive to phase errors, or reasonable frequency errors in the case of locally generated mixing frequencies. However, they are sensitive to misalignment or non-orthogonality of the inphase and quadrature modulated baseband signals in a degree dependent on the modulation technique and data rate. To decrease the data error rate, it is desirable to maintain the non-orthogonality to a low level commensurate with the modulation technique and data rates used.
A typical prior art system is illustrated in a simple block diagram form in FIG. 1. A reference frequency REF is applied to a phase-locked loop (PLL) phase comparator which compares the reference frequency and phase with the output of a divider receiving an input from a voltage controlled oscillator (VCO) to provide an error signal to a low pass filter LF, providing the voltage control to the voltage controlled oscillator. Thus the output frequency and phase is smoothly controlled by programming the divider.
The output of the VCO is used to create inphase (0xc2x0) and quadrature (90xc2x0) components that drive mixers MR1 and MT2, and MR2 and MT1, respectively. An incoming RF signal RF_IN is downconverted by mixers MR1 and MR2 to provide inphase and quadrature components IR and QR of a baseband signal, that is then passed to a demodulator (not shown) for recovery of the data signal therein. In the case of a transmitted signal, the inphase and quadrature components IT and QT of the baseband data signal are mixed with the output of the VCO by mixers MT1 and MT2 and combined to provide the radio frequency output RF_OUT.
FIG. 2 illustrates a prior art system similar to FIG. 1, though with the inphase and quadrature components of the baseband signal IR and QR of FIG. 1 combined by an additional quadrature combiner to provide a single output signal IRM_OUT to the intermediate frequency processor. This structure is called an image rejection mixer. Also the inphase and quadrature components for input to mixers MT1 and MT2 for transmission purposes are derived from an intermediate frequency signal IRM_IN. This structure is called a single sideband upconverter mixer.
Alignment methods and apparatus for I/Q phase and amplitude error correction and image rejection improvement that may be used at the time of circuit fabrication or on chip for use before receiving or transmitting a signal. The alignment method may be used to improve the I/Q phase and amplitude accuracy of a direct conversion transceiver or multi-conversion transceiver, or to improve image rejection in up or down conversion mixers. For I/Q (inphase/quadrature) mismatch correction of an upconverter (MT1, MT2), one can couple a small portion of the output signal from an upconverter to an I/Q mismatch monitoring downconverter whose local oscillator frequency is programmed for monitoring the downconverter to produce a low frequency output whose amplitude is proportional to the unwanted sideband of the upconverter. The higher frequency components at the output of the I/Q mismatch monitoring downconverter are filtered out. For I/Q mismatch correction of a downconverter without a quadrature combiner at the output, a pilot tone of a frequency in the desired signal band is injected into the downconverter input. This pilot tone signal mixes with a quadrature local oscillator frequency to produce inphase and quadrature outputs that interface with analog to digital converters directly. Any inphase or quadrature amplitude or phase mismatch will be detected in the digital signal processor circuit. For I/Q mismatch correction of a downconverter with a quadrature combiner at the output, one can inject a pilot tone in the image band and use a log detector amplifier after the quadrature combiner to monitor the image signal level indicating the inphase and quadrature phase and amplitude levels.